In many instances, a drug's efficacy may be affected by the manner in which it is delivered to the body. Drugs may be administered using a variety of controlled-release or sustained-release technologies. For example, drug pumps, patches, and controlled-release tablet formulations may be used for drug administration.
Many chronic eye conditions require sustained or repeated delivery of drugs to the eye. However, systemic delivery of drugs to the eye may be limited by transfer through the blood-ocular barrier, thus requiring very high systemic doses that may lead to toxicity. Commonly, liquid eye drops may be used to administer drugs to the eye. However, only low molecular weight drugs effectively diffuse through ocular tissue. Thus, not all drugs can be formulated in liquid drop form. Other times, drugs may be administered to the posterior of the eye by intravitreal injection. However, repeated intraocular injections may lead to increased risk of infection or damage to the eye. Ocular implants have also been developed, but their use has been limited due to the small size and shape of the eye, and the technical complexity of controlled long term drug delivery.
Given the benefits of targeted drug delivery and the need for complex and/or extended drug therapies to treat some conditions, improved drug delivery devices are desirable. For example, implantable drug delivery devices with improved drug-release schemes to enable user-controlled or physician-controlled delivery of a single drug, or a combination of multiple drugs or drug doses, would be desirable. In particular, improved implantable drug delivery devices for use in the eye are needed.